Since 1971 scientists have conducted yearly photo-identification studies of a population of southern right whales. The whales gather off Argentina's Peninsula Valdés every year between June and December.

Using detailed photographic information on individual females, researchers have created an annual index that charts the deviation of known whale births from the expected number of calves.

Whale Births

Right whales were given their name by 19th-century whalers, who considered them the right whales to hunt because they float when dead.

Today the ocean mammals live in three groups, which are found in the North Atlantic, the North Pacific, and the sea region around Antarctica known as the Southern Ocean.

There about 300 whales in the North Atlantic and an unknown number in the northern Pacific.

The southern group, about 8,000 strong, is healthy, according to the International Whaling Commission, and growing in number.

Under normal circumstances, a female right whale requires three years between births. But if a calf aborts or dies, a female needs two years to recover, and the interval expands to five years.

"The relatively large number of five-year calving intervals can be explained by whales needing two years to recover from a failed pregnancy," said lead study author Russell Leaper of the International Fund for Animal Welfare.

This evidence does not suggest that whales are failing to get pregnant, but rather that unborn and newborn calves are not surviving.